A flexure pivot formed as a thin material connection between solid material portions of a body is disclosed. The thin connection includes a narrow web bounded by adjacent material-free spaces. A force-transfer mechanism for a force-measuring device, in particular a balance, is also disclosed with at least one flexure pivot of the aforementioned kind. The force-transfer mechanism has a stationary portion, a lever arrangement with at least one lever serving to transmit forces to a measuring transducer, and at least one coupling element serving to introduce an input force into the lever arrangement. The coupling element is stiff against elongation but yields to bending, having at least one thin material connection. The at least one lever is supported on the stationary portion or on a preceding lever by means of a flexible fulcrum pivot having a thin material connection. At least one zone of a thin material connection is delimited by a concave-shaped surface facing material-free spaces.
Flexure pivots in the form of thin material connections meeting the foregoing description are generated by either a localized removal of material or by a forming process. They are distinguished by a high degree of flexibility and small reactive forces for small angular deflections about the pivot axis in combination with a high degree of rigidity against forces and torques acting in other directions. These flexure pivots are used predominantly in precision instruments, and exemplary materials for them are aluminum alloys.
A force-transfer mechanism with a flexure pivot in the form of a thin material connection is often used in force-measuring devices, particularly in balances in which the force generated by a load on the weighing pan is converted into an electrical signal through a transducer based on the principle of electromagnetic force compensation. The purpose of the force-transfer mechanism is to reduce the weight force generated for example by the load on the weighing pan of a balance with a sufficient lever ratio so that the force at the other end of the mechanism can be converted into a measuring signal compatible with the available load range of a force-measuring transducer. The angular deflections of the reduction levers and of their flexure pivots are known to be very small in force-measuring devices that function according to the principle of electromagnetic compensation.
A force-measuring device that meets the foregoing description has a parallelogram with two parallel-guiding members connecting a vertically movable leg of the parallelogram to a stationary leg that forms the fixed portion of the parallelogram. A coupling element that is rigid relative to longitudinal forces and at the same time flexibly bendable introduces the force from the parallelogram into a force-reducing lever mechanism that includes at least one lever and is supported on the fixed portion of the parallelogram. A coupling element is in most cases delimited at each end by a thin material connection defining the point of force introduction into the coupling element. If the lever mechanism has more than one lever, the lever arms following each other in the lever chain are connected in each case by a coupling element. Each lever is supported either on the fixed portion of the parallelogram or on a preceding lever by a fulcrum in the form of a flexure pivot.
A device of this type is described in EP A 0 518 202. A force-measuring device with at least one force-reduction lever and at least one coupling element that is stiff in the lengthwise direction but flexibly bendable is made of a monolithic block of material. The material-free spaces are formed as narrow line cuts traversing the material block. The cutting surfaces of the narrow line cuts, which can be produced by spark erosion, are perpendicular to the plane of rotation of the at least one reduction lever. The material portion that forms the at least one lever is connected to the stationary portion of the material block only through a flexure pivot that forms the lever fulcrum and through a coupling element that applies the force to one arm of the lever. The coupling element and the fulcrum are likewise formed integrally out of the monolithic block.
A weighing transducer based on the principle of electromagnetic force compensation is disclosed in EP A 1 054 242, in which the essential parts, i.e., the parallelogram, the lever mechanism, the coupling elements and the fulcrums, are machined out of a single block of material in a configuration where a stationary base portion of the material block extends into the space between the two parallel-guiding members and forms the fulcrum support for the first reduction lever. At least a part of at least one lever is split up into two levers and at least one coupling element is configured as twin coupling elements that are arranged symmetrically on both sides of a projecting cantilever portion of the stationary base part of the material block. The shaping of the individual components of the weighing transducer from a single material block can be accomplished either by milling or by an erosion process. It is also possible to use a casting process for the production of a weighing transducer of this kind.
The measuring resolution and weighing accuracy achievable with the force- and weighing transducers of the foregoing description are limited because the force-transmitting lever system has a spring characteristic causing a reactive force that opposes a deflection of the mechanism. The spring characteristic can be expressed as a spring constant that is determined primarily by the lever fulcrums and the force-introducing end portions of the coupling elements which have the form of flexure pivots. The main contribution to the aforementioned reactive force comes from the lever immediately ahead of the electromagnetic force-compensation coil. The flexure pivots of the levers and coupling elements are often configured as thin material connections delimited on both sides by concave, arcuate surfaces facing material-free spaces. The arcuate surfaces often have a substantially constant radius, which simplifies the manufacturing process.
An exemplary way of reducing the spring constant of a flexure pivot is to reduce the cross-sectional profile of the thin material connection that forms the flexure pivot. One possibility is to reduce the width of the thin material connection in the direction perpendicular to the plane of rotation of the at least one reduction lever. This concept is described, e.g., in EP A 0 518 202. The reduction in width is likewise achieved by dividing the levers with their fulcrums and coupling elements in the weighing transducer according to EP A 1 054 242. Particularly for the lever fulcrums and/or the end portions of the coupling elements, the splitting-up can be achieved with a dead-end bore hole from the top of a material block (e.g., in accordance with EP A 0 518 202) as disclosed in EP A 1 083 420. A dead-end bore hole can also enter the material block from one of the shorter surfaces that are perpendicular to the plane of rotation of the at least one reduction lever.
As another possibility, it is also possible to make the flexure pivot thinner, i.e., to reduce the profile dimension of the flexure pivot in the plane of rotation of the reduction lever.
In particular, for thin material connections that are delimited by concave surfaces facing material-free spaces with a substantially constant and relatively small radius, the last-mentioned concept leads to a thin flexure pivot with a well-defined center of rotation. However, if a flexure pivot of this kind is exposed to a shock from the outside, the thin portion will easily break. The thinner or narrower the profile of the thin material connection and the stronger the surface curvature towards the adjacent material-free portions, the greater is the risk of breakage.
To solve the foregoing problem, flexure pivots have been disclosed with an elongated shape of the thin material connection. This configuration makes the flexure pivot significantly less sensitive to shock loads from the outside, which may be caused for example by an impact hitting the force-measuring device that contains the flexure pivot. By deflecting sideways, an elongated thin material connection can yield to a shock without breaking, and the lateral deflection is in most cases reversible. A similarly high degree of shock resistance can be achieved with a thin material connection that is delimited on both sides by concave surfaces facing material-free spaces of a constant radius, if the radius is selected large enough. In a further design of a relatively shock-insensitive flexure pivot, the thin material connection is delimited on each side by at least two mutually adjacent concave surfaces facing material-free spaces with approximately constant radii.
The aforementioned concepts for the shape of the thin material connection forming a flexure pivot have the drawback that the center of rotation of the flexure pivot is defined only with a low degree of accuracy.